1. Field of the Invention
The present invention relates to high density magnetic recording, particularly to magnetic particles useful as a coating type high density magnetic recording medium and a process for producing the same.
2. Description of the Related Art
In the field of magnetic recording, the advent of video and digital audio, etc., has led to a demand for high density recording. In longitudinal magnetic recording, which is currently most widely used, if a higher recording density is required, the demagnetizing field within the magnetic layer must be increased, and therefore, any improvement of the recording density is limited. Accordingly, to obtain high density recording, the industry is now carrying out intensive research into the process of perpendicular magnetic recording.
As methods of producing a magnetic layer having a readily magnetization easy axis in the perpendicular direction, there are known a method of forming a thin film of, for example, Co--Cr on a substrate by sputtering or evaporation and a method of coating a magnetic powder of, for example, barium ferrite, together with a binder. The coating method is widely used at present, and therefore, the coating type recording medium is more convenient for practical application.
The coating type perpendicular magnetic recording medium is obtained by using barium ferrite particles, particularly hexagonal platelet barium ferrite particles. In this case, the magnetic particles preferably have an average fine particle size at which they do not exhibit a superparamagnetic property, most preferably an average particle size of 0.01 to 0.3 .mu.m, as described in, for example, Japanese Unexamined Patent Publication (Kokai) No. 56-125219, which states that the superiority of perpendicular magnetic recording over longitudinal magnetic recording becomes clear in the region where the recording wavelength is 1 .mu.m or less, and in this wavelength region, to obtain a satisfactory recording and reproduction, the crystal particle size of the above ferrite is preferably 0.3 .mu.m or less. If this crystal particle size is about 0.01 .mu.m, a required strong magnetism is not exhibited, and therefore, the preferable crystal particle size is about 0.01 to 0.3 .mu.m.
Next, as excellent a dispersibility as possible must be obtained, as described in, for example, Japanese Unexamined Patent Publication (Kokai) No. 56-155022, which states that, if a uniform dispersion is not obtained when formed into a coating material, a good recording medium cannot be obtained, and therefore, the individual particles must not be sintered and agglomerated at least during the preparation of a magnetic powder.
The saturation magnetization must be as large as possible, as described in, for example, Japanese Unexamined Patent Publication (Kokai) No. 56-149328, which states that, the magnetoplumbite ferrite to be used for the magnetic recording medium material must have as large a saturation magnetization as possible.
Further, the coercive force must be 300 to 1500 Oe, in association with the reproduction output and head properties, and the degree of dependence of coercive force on temperature must be as small as possible, as described in, for example, Japanese Unexamined Patent Publication (Kokai) No. 56-149328, which states that a magnetoplumbite ferrite exhibiting a coercive force value of 500 to 1500 Oe when formed into fine powder must be obtained. The coercive force must be large for a higher density and reproduction output, but a practical upper limit thereof is about 1500 Oe, due to the problem of head properties. Also, for example, Japanese Unexamined Patent Publication (Kokai) No. 62-132732 states that the coercive force of a magnetic recording medium greatly affects the electromagnetic converting characteristic, and fluctuation of the electromagnetic converting characteristic will cause a corresponding fluctuation of the recording, reproduction, and erasing characteristics. Namely, when a magnetic recording medium having a coercive force largely dependant on temperature is used at the places where the environmental temperatures are remarkably different, defective recording, lowering of reproduction output and a defective erasing of a recording will occur, whereby the function as a magnetic recording medium will be remarkably lowered.
As processes for the production of the hexagonal platelet barium ferrite particles for magnetic recording, the following three processes are known:
(1) The process of obtaining barium ferrite particles by mixing an alkali and an alkali carbonate into an aqueous solution containing a ferric salt and a barium salt, while stirring, to obtain a coprecipitate of ferric hydroxide and barium carbonate having a pH value of 10 or higher, thoroughly washing the coprecipitate with water and then drying, followed by a heat treatment at about 900.degree. C. (refer to Japanese Unexamined Patent Publication (Kokai) No. 56-60002). PA1 (2) The process of obtaining barium ferrite particles by heating an alkali solution having a pH value of 10 or higher and composed of dissolved or coprecipitated Fe.sup.3+ and Ba.sup.2+ to a temperature of 100.degree. to 374.degree. C. in an autoclave, to react and form a barium ferrite precursor (barium ferrite with an incomplete crystallinity and magnetic characteristic), followed by washing, drying and calcining (generally at 800.degree. C. or higher) the reaction product (refer to, e.g., M. Kiyama, Bull. Chem. Soc. Jpa., 49 (1976) 1855; and Japanese Unexamined Patent Publication (Kokai) No. 60-12973). PA1 (3) The process of mixing and melting the barium ferrite constituent starting materials such as BaO and Fe.sub.2 O.sub.3 with a glass forming material such as B.sub.2 O.sub.3, solidifying the molten mixture by quenching, then heat treating the solidified product to precipitate barium ferrite in the glass material, and dissolving the matrix to extract the barium ferrite particles, followed by washing with water and drying (refer to, e.g., Kenjo, Ido, Nikkei New Material, Apr. 28, 1986, page 52, and Japanese Unexamined Patent Publication (Kokai) No. 56-67904).
It is known in the art that barium ferrite particles for magnetic recording, satisfying the required average particle size and dispersibility conditions can be provided by the production processes described above.
Also, it is possible to obtain a required coercive force value by replacing a part of Fe in barium ferrite with another element, such as Co--Ti (e.g., refer to J. Smit H. P. J. Wijn Ferrites, 1959, page 208). Also, a method has been proposed of controlling a coercive force by making the magnetoplumbite type ferrite phase represented by MO.nFe.sub.2 O.sub.3 and the spinel type ferrite phase represented by M.sup.+ O.Fe.sub.2 O.sub.3 coexist (Japanese Unexamined Patent Publication (Kokai) No. 56-118304), and the method of controlling magnetic characteristics by modifying only the surfaces of the magnetoplumbite type ferrite particles with the spinel type ferrite (Japanese Unexamined Patent Publication (Kokai) No. 62-139123).
Nevertheless, in the method of controlling a coercive force by the replacement described above, a drawback arises in that lowering a saturation magnetization compared with the barium ferrite before replacement is incurred. Further, with respect to the thermal stability of a coercive force, coercive force of the barium ferrite before replacement is increased as the temperature is elevated, but after the replacement, the degree of dependence of the coercive force on temperature is further increased, and in some Examples shown in Japanese Unexamined Patent Publication (Kokai) No. 62-155504, reaches 0.61%/.degree. C. This deterioration in magnetic characteristics caused by the replacement makes magnetic recording very difficult, and is an obstacle to a practical application of the particles.
The particles, only the surfaces of which are treated as seen in Japanese Unexamined Patent Publication (Kokai) No. 62-139123 have a drawback in that the magnetic characteristics are changed with an elapse of time.
In view of these problems, the present invention provides a material for magnetic particles for perpendicular magnetic recording, which maintains a high value of saturation magnetization, especially a value of 60 emu/g or higher, with a coercive force controlled to an adequate value (300 to 1500 Oe), and at the same time has an excellent coercive force thermal stability, especially a degree of dependence of coercive force on temperature of 0.2%/.degree. C. or lower, without a change in the magnetic characteristics with an elapse of time.